Particle detectors are devices to detect, track and/or identify radiation or particles and find wide applications throughout particle physics, biology as well as medical technology.
Particle detectors exploiting in the process of ionization and charge multiplication in gases have been in use with continued improvements ever since Rutherford first employed a gas-filled wire counter to study natural radioactivity more than a century ago. Techniques for designing detectors of increased physical size and/or increased robustness to discharges and/or enhanced spatial resolution continue to be an active field of research in the detector community today.
Gaseous detectors typically collect the electrons released by ionising radiation and guide them to regions with a strong electric field, thereby initiating an electron avalanche. The avalanche is able to produce enough electrons to create a current or charge large enough to be collected on a readout device and analysed by means of readout electronics. The collected electron charge may indicate the charge, energy, momentum, direction of travel, and other attributes of the incident particles or radiation.
Conventionally, the large amplification field necessary to initiate and support the electron avalanche has come from a thin wire at a positive high voltage potential. This same thin wire has conventionally been employed to collect the electrons from the avalanche and to guide them towards the readout electronics. More recently, so-called MicroPattern Gas Detectors (MPGDs) such as the MicroMesh Gaseous Structure Chamber (MicroMegas) and the Gas Electron Multiplier (GEM) that employ semiconductor fabrication techniques have made it possible to mass-produce detector devices in an impressive variety of geometries while at the same time permitting small avalanche gaps and hence rapid signal development in combination with fast readout and high reliability.
In MPGDs, the electrons generated in the amplification process are conventionally collected on metallic readout pads or strips that are arranged in a predetermined pattern on a semiconductor substrate and electrically connected to fast readout electronics by means of wire connections. For the example of a MicroMegas detector, this configuration is described in U.S. Pat. No. 6,133,575, whereas a GEM detector is described in U.S. Pat. No. 6,011,265.
A serious problem generally encountered in gas-filled proportional chambers is sparking induced by heavily ionising particles that may trigger large number of electrons. Amplified by the avalanche process, they may reach the Raether limit of a few 107 electrons, and may evolve into a discharge. This is a particular challenge for modern accelerators with high luminosities, which may produce high count rates from slowly-moving recoils originating from elastic scattering and/or low-energy hydronic debris from nuclear breakup.
Sparks can lead to a temporary high voltage breakdown and hence may give rise to unwanted detector dead times, in which the detector needs to recover and no new events can be detected. Sparks may also damage the readout pads and/or readout electronics. To reduce detector dead times and to avoid damage, most detectors employ additional protecting circuits that interface the readout strips or pads with the front end electronics. These protection circuits add to the complexity of the device and require additional wiring, which conflicts with the desire to form detector devices with ever more and ever smaller readout pads.
Efficient protection against discharges is particularly important in modern grid pixel (“GridPix”) detectors, in which the set of readout pads and readout circuits is replaced by a semiconductor readout board or pixel chip integrated into the detector structure. The readout chip of a grid pixel detector may double as the anode of the detector device, and may incorporate a large number of square pixels each connected to its respective pre-amplifier, discriminator and digital counter. An example of a grid pixel detector in which a MicroMegas detector is placed directly on a pixel chip is described in further detail in P. Colas et al., Nucl. Instr. and Meth. A 535 (2004), p. 506.
In comparison with conventional MPGDs, grid pixel detectors have the advantage that they integrate much of the readout circuitry, thereby allowing to form smaller more compact detector devices and to enhance the spatial detector resolution. However, this structure is particularly sensitive to discharges. Even a local discharge will not effect only a single readout channel, as is the case in conventional MPGDs. Rather, discharges may result in a local melting or evaporation of chip material or a breakdown of electronic circuitry that will affect the entire chip. Since the chip is integrated into the detector structure, often the entire structure will need to be replaced.
In order to protect the chip of a grid pixel detector from discharges, a high-resistive layer of 5 μm up to 25 μm of amorphous silicon may be deposited on the chip, as described in I. Bilevych et al., Nucl. Instrum. Meth. A 629 (2011) 66-73. When a discharge propagates through the gas, a charge builds up at the surface of the silicon, thereby locally reducing the electric field and spreading the charge both in time and location. However, this high-resistive layer alone may not be sufficient to protect the chip against discharges in the harsh background environments encountered at large energies and high luminosities.
What is needed is a detector device that allows to provide an efficient protection of the readout board against discharges, and at the same time allows for an easy repair at low costs if discharges should nevertheless occur.